The invention relates to a novel process for preparing ammonium heptamolybdate.
Ammonium heptamolybdate (referred to hereinafter as AHM) is a known intermediate for the preparation of molybdenum metal powder and of molybdenum-containing catalysts.
In principle, ammonium heptamolybdate (referred to hereinafter as AHM) is prepared via the dissolution of technical molybdenum oxides, which are obtained by roasting molybdenum sulphide concentrates, in water or alkaline solution. The resulting solution is, for example, purified via liquid-liquid extraction or using a solid ion exchange resin. The purified solution is generally concentrated by evaporation and, after the required pH has been established, for example by adding mineral acids, AHM is crystallized by means of cold crystallization.
In a further route, the prepurified ammonium molybdate solution is reprocessed by acidification to give an intermediate, for example ammonium polymolybdate. The intermediate is dissolved in ammoniacal solution under hot conditions, and AHM is subsequently crystallized by cooling.
A process for preparing AHM is described in SU-A-1,723,042. The process consists in digesting technical molybdenum oxide with water and ammonia solution at pH 6.0-6.8, and obtaining a solution with a content of about 20-24% MoO3. This solution is subsequently filtered through combined fabric and membrane filters at about 20-30° C. and concentrated by evaporation at about 50-70° C. down to an MoO3 concentration of about 36-40%. Thereafter, the AHM product is crystallized by cooling. Disadvantages of this process are the high energy consumption in the concentration of the molybdenum solution, the high content of impurities in the AHM owing to the lack of a purification step for the removal of dissolved impurities, and the low product yield as a result of the molybdenum losses with the mother liquor.
U.S. Pat. No. 4,079,116 describes a further process for preparing AHM. This comprises the leaching and filtering of technical molybdenum oxide with water and the purification of this solution by means of an ion exchange process to free it of cationic impurities. Subsequently, the residue of the water leaching is combined with the purified solution and digested with ammonia solution. From the resulting solution, ADM is crystallized by means of an evaporative crystallization. After setting the pH, for example with carbon dioxide, the ADM mother liquor is finally conducted to the AHM cold crystallization. In a second embodiment of this patent, the solution of the ammoniacal digestion is concentrated by evaporation under reduced pressure at about 55° C. for concentration and setting of the pH, and then conducted to the AHM cold crystallization. A disadvantage in these processes is that an energy-intensive concentration of the molybdenum solution by evaporation is again carried out, and that the AHM product has high impurity contents, since the purification stage here does not enable the removal of anionic impurities and alkali metals. A further disadvantage of the process is that a high ammonia excess based on the AHM product is used, which has to be recovered by a complicated process.
The publication “The use of solvent extraction for the production and recovery of high-purity ammonium paramolybdate from normal alkali molybdate solution” (Journal of the Less-Common Metals (1974, 36 (112), 111-116) describes the preparation of AHM. In this process, technical molybdenum oxide is digested with sodium hydroxide solution, and the resulting sodium molybdate solution is purified by means of a liquid-liquid extraction. In the extraction, the molybdenum is taken up selectively by an amine-containing organic phase and, after a water wash, stripped with 15.7 molar ammonia solution in excess at a quantitative ammonia:molybdenum ratio (referred to hereinafter as NH3:Mo ratio) of about 9.0. The resulting pure ammonium molybdate solution with 250 g/l of MoO3 is concentrated by evaporation and sent to an AHM cold crystallization. A disadvantage of this process is again the high energy consumption of the concentration of the molybdenum content required here and of the setting of the NH3:Mo ratio of 0.9 to 1.4 needed for the AHM crystallization. Moreover, the high ammonia excess based on the AHM product has to be recovered by a complicated process.
The article “Production of ammonium paramolybdate from nonstandard molybdenite concentrate by extraction” (Nauch. Tr., Irkutsk. Nauch.-Issled. Inst. Redk. Tsvet. Metal (1972), No. 27, 138-145) describes a process for preparing AHM. In this process, technical molybdenum oxide is digested with soda. The resulting sodium molybdate solution is acidified down to pH 1-2 and the molybdenum is extracted with trioctylamine. The molybdenum loading of the organic phase is about 140 g/l. The reextraction is effected using 15-20% ammonia solution, such that the stripping solution contains 180-200 g/l of Mo and an NH3:Mo ratio of 4.3 to 5.8. By means of a subsequent acidification with a mineral acid, ammonium polymolybdate is crystallized and removed. Thereafter, the crystals are dissolved with ammonia solution and recrystallized to give AHM. A disadvantage of this process is that, again, no direct AHM cold crystallization from the stripping solution can be carried out, since the NH3:Mo ratio of the solution has much too high a value therefor. In order to achieve the required ratio of 0.9 to 1.4, an ammonium polymolybdate intermediate is prepared by means of additional process steps and dissolved again. Here too, the high ammonia excess based on the AHM product has to be recovered by a complicated process.